Lecture 5 Nitrogen metabolism, proteins as macronutrients
Sunday, 23 September 2012
Purines and pyrimidine's have no dietary requirement, the human body synthesises them itself
by use of salvation pathways.
Excess dietary nucleic acids my cause gout eg insoluble uric acid.
20 Commonly found in proteins
Humans can synthesise about half
Remainder must be present in diet
Surplus dietary protein provide amino groups for biosynthesis, surplus carbon skeletons
oxidised or otherwise metabolised.
Surplus ammonia detoxified to UREA.
Acid denaturation, hydrolysis and pepsin digestion in stomach. Enzymes present in stomach
are inactive precursors in pancreatic secretions.
Enterokinase on membrane of enterocytes activates trypsinogen
In small intestine:
Trypsinogen --> trypsin
Chyotrypsinogen --> chymotrypsin
Proelastase --> elastase
Peptide further degraded by CAP, CPB
Amino acids and very small peptides absorbed by intestinal cells (brush border)
Peptidases break down peptides to amino acids
Active transport from the intestinal lumen into the epithelial cells
Facilitated diffusion into blood from epithelium
Amino acid taken up by cells for protein synthesis or metabolism
A number of protein degradative pathways exist in cells but all involve proteinases.
Cellular cytoplasm proteinases are catalysed by calpains or proteasomes
Lysosomal proteinases are called cathepsins, recycle amino acids
Digestibility of protein:
Denaturation by heat (cooking) improved digestibility
Globular protein are digested and absorbed completely as well most fibrous proteins with the
exception of a few
Negligible residual nitrogen in faeces
Some foods rich in proteinases inhibitors which may impair digestion
Turnover of body protein:
1-3% body protein degraded daily, this yields about 300g of amino acids from turnover of
endogenous amino acids
Another 100g is obtained daily from a standard westernised diet